Summa Theologiae Moralis, vol. I: De Principiis

(On Sins of Ignorance).

S. Th. I-II, q. 76; De Malo q. 3, a. 6, 7, 8.

464. On the part of reason the cause of sin is ignorance and inconsideration of the intellect, not any whatsoever but that which removes the knowledge (whether universal of right or particular of fact or circumstance) which, if it were present or more fully present, sin would not be done. Whence it is not the cause of sin of itself, but accidentally insofar as it removes that which would impede sin. — But if someone is so disposed that without ignorance he would nevertheless sin, the ignorance would not be the cause of sin: such a one does not sin from ignorance or because of ignorance, but sins with ignorance or ignorantly. Thus if the ignorance is concomitant, or even if it is affected but precisely for the sake of sinning more freely.

For every sin, even one done from malice, ignorance or error is required in the last practical judgment, by which reason erroneously judges such an act to be illicit or to be good for him here and now¹; but sins of ignorance are those which also proceed from a culpably erroneous speculative judgment².

465. Ignorance can be sin (the contrary was condemned in Coelestius and Abelard), not indeed of itself: for it is an intellectual evil, not a moral one, but by reason of a preceding negligence in learning or an actual one in considering (Lk. XII, 48; I Tim. I, 13). Nevertheless not every ignorance is sin: for invincible ignorance, which someone could not repel, is not sin, as is evident from prop. 68 of Baius, and 2 by Alex.

VIII condemned³.; but vincible ignorance is only sin if it is about those things which someone is bound to know, or which are necessary for directing his own acts and without which he cannot rightly exercise the due act.

Whence for sin it does not suffice that ignorance be voluntary, but moreover it is necessary that it be culpable. For which three things are required: 1) in order that it be voluntary 1) that one could have removed it, i.e. that he had the means for it, and 2) that he in some way adverts to it; but in order that it be culpable, moreover 3) that one was bound to prevent the ignorance, i.e. to use means lest ignorance follow: thus ignorance of medical science is not culpable for a farmer and a lawyer, but for a physician.

All however are bound to know a) the articles or fundamental truths of faith, b) the universal precepts of law, namely of the Decalogue and of the Church; c) but each individual [is bound to know] those things which pertain to his state or office, at least those which are common and often occur.

Moreover ignorance can be a special sin. — Vincible ignorance 1) of a thing which we are bound to know habitually or by reason of itself (ignorance of right) is a special sin of omission, distinct from the sins which follow from it, because the ignorance itself is morally evil in itself, and because the sinner thus violates two diverse precepts.

As e.g. if one were voluntarily ignorant of the incarnation or of the malice of adultery or of the five precepts of the Church, and therefore blasphemed Christ, or committed adultery, or ate meat, he would be guilty of a twofold sin.

On the contrary 2) ignorance of a thing which one is bound to know, not indeed permanently and by reason of itself, but transiently and because of the work to be done here and now (ignorance of a particular fact, as e.g. of a circumstance of the act) is not a sin distinct from the sin which follows from it, or to which it exposes, because it is not evil except by reason of the evil effect, and therefore the sinner acts against only one precept and one virtue.

Thus negligence in inquiring whether the present day is a day of fast (even if it were not a day of fast), because from the same precept he is bound both to fast and to inquire.

466. Principles. I. Invincible and antecedent ignorance of itself totally excuses from the whole sin the work which is done from it, because it renders it involuntary (Gen. IX, 21); accidentally it does not entirely excuse if it is not total but partial, because only the circumstance is ignored which, if known, sin would not be done, and yet knowingly sin is done, as e.g. the killing of a man by one who does not know him to be a father (Gen. XXXVIII). Whence it excuses insofar as it is ignorance.

II. Invincible and concomitant ignorance does not entirely excuse: it excuses from the sin of the external work which is done with it because it renders it non-voluntary; but it does not entirely excuse the agent himself, because it does not take away all will of sinning or internal affection for sin, since the work would nevertheless be done if the ignorance were not present, as e.g. he who kills an enemy, thinking him to be a beast, would nevertheless kill if he knew. Such ignorance neither excuses namely entirely, nor accuses.

III. Vincible and consequent ignorance of those things which someone is bound to know, if it is the cause of sin which otherwise would not be done, does not excuse entirely from the sin the work which is done from it, because it remains voluntary (since the ignorance itself is voluntary), but it diminishes it because it renders it involuntary in a qualified sense. Whence it accuses, as being voluntary, and does not excuse from the whole but from so much, as e.g. wounding from drunkenness.

IV. Vincible but affected ignorance, and excited precisely for the sake of sinning more freely, in no way excuses from sin but rather increases it, because it proceeds from a greater will of sinning. Whence it simply accuses.